The present invention is a composite wind turbine blade construction and a method for manufacturing the same. Wind turbines convert kinetic energy in the wind into electrical energy. A typical wind turbine configuration is a three-bladed Horizontal Axis Wind Turbine (HAWT). Turbines of this configuration typically have a rotor with three wind blades with aerodynamic profiles attached to a central hub or rotor. The rotational axis of the rotor is approximately horizontally positioned and movable to be approximately aligned with the direction of the wind. Wind passing over the surfaces of the wind blades generates lift, which the rotor assembly converts to torque. The rotor is connected to a driveshaft extending through the front end of a nacelle, or enclosure. In certain wind turbine designs, a gearbox inside the nacelle increases the angular velocity of the rotation of an output shaft that drives an electrical generator converting the rotary motion into electricity. In alternative wind turbine designs, the driveshaft may operate a hydraulic pump or other hydraulic system to drive the electrical generator. The nacelle and rotor assembly is typically supported by a tower.
The efficiency and cost effectiveness of a HAWT depends in part on the average wind velocity and the area swept by the wind blades as the rotor rotates. As the area swept by the wind blades changes with the length of the blades, the length of commercial wind blades has increased. For higher wind velocity locations, longer wind blades may provide an increased return on investment. For lower wind velocity locations, longer wind blades may be required to provide a commercially viable investment.
Desirable locations for wind turbines are those with high average wind velocities. As the demand for wind power has increased many of the most accessible high average wind velocity locations have been utilized. Continuing demand has made more remote locations attractive despite lack of infrastructure to support transportation of wind turbine components, transport and operation of erection equipment, ongoing maintenance operations and access to existing electrical grids. Consequently, accessible locations with lower average wind velocities are also becoming commercially desirable as demand for wind power increases. The need to effectively utilize remote high average wind velocity locations and accessible lower average wind velocity locations increases the need for longer, larger wind blades.
In the past, the length of the wind blades has been limited by the capabilities of the structure and materials to support the weight of the blades and to withstand static and dynamic loads in service. Additionally, the breadth of the wind blades increases with the increase in length. Longer, wider blades require more material, which increases the weight of the wind blades. The length, breadth and weight increases impose constraints on transportation. In particular, overall dimensions for transport are limited by the quality of access roads, the clearance height of overhead obstructions on the transport path and weight limits on bridges. Transport constraints increase the need for blades of lighter weight materials and blades that can be transported in pieces and/or fabricated on-site while material capabilities drive a concomitant need for higher strength structures and materials.
Typical of many prior composite wind blades is a construction of a box spar assembly having shear webs forming the sides and spar caps forming the upper and lower surfaces. Skin halves are affixed to the upper and lower surfaces and joined at the leading and trailing edges of the wind blade. Thin composite materials in the shear webs, spar caps and skins over lightweight core materials such as balsa or foam materials yield high strength, light weight structures. This prior construction requires molds to form the skin halves. These molds are large, expensive, and subject to the same transportation constraints discussed above, rendering this construction method uneconomical for on-site fabrication of wind blades. Variations of prior methods employing multiple-piece shear web/spar cap spar assemblies and multiple piece skin segments have been proposed.
An alternate prior wind blade construction uses a composite tubular spar affixed to skin halves. Additional shear webs have been used with this construction. However, this construction is subject to the same constraints and economies as conventional shear webs/spar caps/skins construction.
Multiple-piece wind blades where each piece contains core structure and outer skins in a completed subassembly have been proposed for simplified transport and on-site assembly. However, the joints between blade segments of the prior multiple-piece wind blades can be weak points relative to the required service loads. Strengthening of the joints increases material weight and incurs additional ongoing monitoring and maintenance requirements for the blade structure. Additionally, the joints typically are exposed to the environment and prone to wear from the environment, wind-borne particles, and other damage.
Prior one-piece wind blades have attempted to include higher strength fibers in the spars and skins, utilize bend-twist coupled composite structures to reduce lift in extreme wind load conditions, and various static and dynamic aerodynamic modifications to the wind blade surfaces. Additionally, wind blade manufacturers in the past have provided a limited range of fixed wind blade configurations to minimize engineering time and costs, manufacturing tooling and equipment, and to minimize testing for validation. Each configuration was designed to service the widest possible range of wind regimes. As a result, the prior wind turbine blades were rarely optimized to the wind conditions to be experienced by each specific wind turbine installation and not cost efficient for all installations. There remains a need for a wind turbine blade that is efficiently transported and provides efficient and cost effective service over a range of blade lengths and wind regimes. There further remains a need for the capability to optimize the structural properties of a wind blade configuration to a specific installation without resulting in expensive engineering and manufacturing changes.
A wind turbine blade is disclosed comprising a plurality of longitudinal composite members each having a fiber and resin layer around a predetermined cross-sectional shape and each comprising at least one longitudinal outer surface and at least one longitudinal mounting surface, each of the outer surfaces of the plurality of longitudinal composite members corresponding to a different portion of a desired airfoil shape, the plurality of longitudinal composite members assembled such that the outer surfaces of the composite members form at least a majority of the airfoil shape. The mounting surface of one longitudinal composite member may be positioned opposite the mounting surface of an adjacent longitudinal composite member with a fiber and resin layer there between.
The fiber and resin layer of the longitudinal composite members may include a braided sleeve comprising fibers selected from a group consisting of glass fiber, carbon fiber, and a combination thereof. The braided sleeve may comprise fibers having a bias angle in one direction less than the bias angle in the other direction. Alternatively or additionally, the fiber and resin layer of the longitudinal composite members may include a triaxial braided sleeve. The fiber layer of the longitudinal composite members may comprise axial features capable of intermeshing with an adjacent fiber layer in the assembly.
An outer skin may be positioned over the assembly of longitudinal composite members, the outer skin comprising a fiber and resin layer. The fiber in the fiber and resin layer of the outer skin may comprise a continuous, contoured braided sleeve.
The wind turbine blade may include at least one of the plurality of longitudinal composite members having a variable cross section along its length. Alternatively or in addition, each of the longitudinal composite members may comprise a plurality of segments joined end to end. Segments of the longitudinal composite members may be less than about 40 feet in length. In one example, at least one of the segments comprises a constant cross sectional shape along its length.
Also disclosed is a method of manufacturing a wind turbine blade comprising providing a plurality of longitudinal composite members each having a fiber layer around a predetermined cross-sectional shape and comprising at least one longitudinal outer surface and at least one longitudinal mounting surface, each of the outer surfaces of the plurality of longitudinal composite members corresponding to a different portion of a desired airfoil shape, and assembling the plurality of longitudinal composite members such that the outer surfaces of the composite members form a least a majority of the airfoil shape. The method may comprise providing an outer skin over the assembly of longitudinal composite members comprising a fiber and resin layer.
The method may include, prior to the step of assembling the plurality of longitudinal composite members, infusing resin into the fiber layer around each longitudinal composite member, and curing the resin.
In the method, the step of assembling the plurality of longitudinal composite members may comprise infusing resin into the fiber layer around each longitudinal composite member, then assembling the longitudinal composite members such that the mounting surface of one longitudinal composite member is opposite the mounting surface of an adjacent longitudinal composite member with a fiber and resin layer there between, and curing the resin.
The step of providing a plurality of longitudinal composite members may include over-braiding the fiber layer around the predetermined cross-sectional shape.
Where each of the longitudinal composite members comprises a plurality of segments, the method may further comprise the step of assembling the segments to form the plurality of longitudinal composite members. Additionally, prior to the step of assembling the plurality of longitudinal composite members, the method may include the steps of transporting the plurality of segments to a desired location, and assembling the segments to form the plurality of longitudinal composite members at the desired location. The method may comprise providing an outer skin over the assembly of longitudinal composite members comprising a fiber and resin layer.
A kit for making a wind turbine blade may comprise a plurality of segments adapted to be joined end to end to form longitudinal composite members, each having a fiber layer around a predetermined cross-sectional shape and comprising at least one outer surface, each of the outer surfaces of the plurality of longitudinal composite members corresponding to a different portion of a desired airfoil shape, the plurality of longitudinal composite members capable of being assembled such that the outer surfaces of the composite members form at least a majority of the airfoil shape.
The kit may include a fiber layer adapted for enveloping the longitudinal composite members after assembly for forming an outer skin comprising a fiber and resin layer. The fiber layer may be a continuous, contoured braided sleeve.
The longitudinal composite members may be adapted to be assembled by infusing resin into the fiber layer around each of the plurality of longitudinal composite members.
The above summary is not intended to describe each embodiment or every implementation of the present invention. A more complete understanding of the invention and its advantages will become apparent by referring to the following detailed description and claims in conjunction with the accompanying drawings.